In the field of vehicle suspensions, the phrase "quarter car suspension" refers to the components of the vehicle suspension relating to one of the four wheels of the typical automotive vehicle. These components include the particular wheel with a tire that is in contact with the road, a spring that transfers the road force to the vehicle body (sprung mass) and suspends the vehicle body, and a damper or actuator that reduces undesirable relative movement between the vehicle body and wheel. The complete suspension system of an automotive vehicle comprises four quarter car suspensions.
In recent years, vehicle manufacturers have dedicated significant effort to developing suspension systems responsive to the driving conditions of the vehicle. This effort is triggered by desire to incorporate the best features of soft and stiff suspension systems into a single vehicle suspension system. The best feature of a soft vehicle suspension is the smooth ride it provides for the vehicle passengers. The best feature of a stiff vehicle suspension is the increased handling performance it provides for the vehicle.
The theory of semi-active suspension systems is to selectively switch between stiff suspension and soft suspension in response to the particular driving conditions of the vehicle. Selection between stiff suspension and soft suspension may be obtained by altering the damping force of the suspension system, e.g., a greater damping force for a stiffer suspension and a lower damping force for a softer suspension. With correct control of suspension damping force, a vehicle can provide both optimum driving comfort and optimum handling performance.
The theory of active suspension system controls is to provide an actuator force to the suspension system to reduce wheel hop and improve vehicle body attitude control beyond that achievable by damping forces alone. The actuator force is applied in equal and opposite directions between the wheel and vehicle body. Active and semi-active suspension systems can be commonly referred to as variable force suspension systems.
Difficulties in designing variable force suspension systems lie partially in system controls. A suspension system may, at any given time, be said to have a state. The suspension system state for a particular quarter of the vehicle includes the position of the vehicle body (the sprung mass), the position of the wheel (the unsprung mass), the velocity of the sprung mass, and the velocity of the unsprung mass. From these four components, the other characteristics of the quarter car suspension system may be determined. For example, the relative velocity between the sprung mass and the unsprung mass is equal to the velocity of the sprung mass subtracted by the velocity of the unsprung mass. The relative position of the sprung mass and unsprung mass is equal to the position of the sprung mass subtracted by the position of the unsprung mass. The relative velocity between the sprung and unsprung masses and/or the relative position of the sprung and unsprung masses may be included in what is referred to below as the relative system state.
The state of the quarter car suspension system is difficult to predict because the road surface is always changing and is, itself, not predictable. Directly measuring the suspension system state requires many sensors, and is difficult to do. What is desired is a controller for a variable force suspension system that estimates the quarter car suspension system state based on as few sensors as possible while keeping the error between the actual and estimated state within reasonable limits. It is also desired that the suspension controller not be affected by error in the estimated state.
There is also the desire to eliminate suspension system chatter. Chatter occurs when the suspension control system is on the border between commanding high actuator force and low damping force and constantly switches between the two.